Oxide glasses having low glass transformation temperatures

ABSTRACT

PbO-B 2  O 3  -Ag 2  O glasses having a preferred weight ratio range of 76-88:10-15:2-10 have glass transition temperatures of less than 300° C. and are especially useful for adhering together ceramic substrates such as for attaching integrated circuits to ceramic substrates. The glasses can be formulated with Ag particulates and organics and applied as pastes to provide conductive adhesives.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to adhesive glasses and more particularly toPbO-B₂ O₃ -Ag₂ O glasses with low glass transformation temperatures(T_(g)).

2. Description of the Previously Published Art

Current conductive die attach adhesives which are used for attachingintegrated circuits to ceramic substrates are essentially a mixture ofsilver and glass powder in an organic vehicle. They require firing attemperatures in the range 420°-450° C. to achieve strong bonding so asto ensure reliability of the integrated circuit package assembly. Theprimary reason for requiring such firing temperatures is to achievesufficient flow in the glass component of the adhesive to allow wettingof the integrated circuit chip and the ceramic substrate. These highfiring temperatures can result in degradation of the integrated circuitchips and migration and oxidation of Ni on the metallization of thesubstrate. See, for example, Owens, N. L., Microelectronic Manuf. andTest., April 1986, pp. 6-7, at p. 7, which reviews nickel migrationduring organic burn-off.

To lower the firing temperature and still achieve good bonding, glasseshaving lower glass transformation temperatures (T_(g)) or having morefluid properties at lower temperatures are required.

Most glasses used in conductive die-attach adhesives are PbO-B₂ O₃ basedglasses with various oxide additives such as SiO₂, ZnO, Bi₂ O₃, and BaO.These glasses have a T_(g) typically above 300° C. Alkali metal oxides(Na₂ O, K₂ O) and thallium oxide can be added to the glass melt to lowerthe T_(g), but such oxide additions are known to result in glasseshaving poor resistance to moisture and, therefore, not acceptable asdie-attach adhesives. See Takamori, T., "Solder Glasses", a chapter atpages 173-255 in Treatise on Materials Science and Technology, Vol. 17,Glass II, Edit. M. Momozawa and R. H. Doremus, Academic Press (1979).

Certain other oxides lower the T_(g) of PbO-B₂ O₃ glasses but areavoided for various reasons. CdO and BaO present additional toxicityproblems; Bi₂ O₃ attacks platinum crucibles conventionally used inpreparing the melts. PbO-B₂ O₃ based glasses will dissolve oxidecrucibles such as silica, alumina, clay, porcelain, etc. It is thereforeimportant to match the glass to the melt container and vice-versa.

To meet the stringent requirements for die attachment, aside from lowerT_(g) and considerations above mentioned, the glasses must also beresistant to crystallization during the firing step. This is becausecrystallization of the glass can lead to abrupt increases in viscosityand, hence, prevent flow and wetting of the integrated circuit andceramic substrate.

Prior patents disclose various glass compositions, but they provide nosuggestion of the low Tg PbO-B₂ O₃ -Ag₂ O glass composition which arefound effective here for die attach adhesives.

U.S. Pat. No. 3,711,328, Wada et al. (1973), discloses a glass fritconsisting essentially of 0.1-17 weight % Ag₂ O and 83-99.9% of a basecomposition consisting essentially of 60-80% PbO, 8-16% B₂ O₃, 12-24%ZnO. The basic glass composition has ZnO as a required component. InExample 12 of the patent, PbO is 64.7 weight %, B₂ O₃ 11.6%, ZnO 13.7%,Al₂ O₃ 7.0%, and Ag₂ O 3.3%. It is used in making a resistor paste withno suggestion for use as an adhesive.

U.S. Pat. No. 4,401,765, Dietz et al. (1983), discloses a silver-filledlead borosilicate glass metallizing paste for bonding silicon dies toceramic substrates. Ag₂ O is absent.

U.S. Pat. No. 4,446,059, Eustice (1984) discloses a conductivecomposition containing lead borosilicate, Ag₂ O, and Ag powder for useas automotive window defoggers. Proportions of ingredients for suchcomposition are stated in column 3 of the patent as (with respect to theinstant invention), 60-98.5% Ag, 20-1% lead borosilicate, and 5-60% of ablend of Ag₂ O and B₂ O₃ as a colorant. This is a physical mixture ofthe ingredients and does not disclose the homogenous compositionas setforth herein.

U.S. Pat. No. 4,636,254, Husson et al. (1987), discloses a silicon dieattach paste consisting of a physical mixture of Ag flake 65.38 weight%, lead borate glass frit 17%, Ag₂ O 2.62%, balance organics to make100%. The Ag flake is 50% 200 mesh, 50% 100 mesh. We differ from Husson,inter alia, in that our Ag₂ O is an integral component of our glass,whereas in Husson the Ag₂ O is in physical mixture.

U.S. Pat. No. 4,699,888, Dumesnil et al. (1987), discloses adhesiveglass pastes containing 75-85 weight % Ag powder and a glass containingPbO, B₂ O₃, SiO₂, Cu₂ O, a metal fluoride, and optionally ZnO, Bi₂ O₃.The materials were powdered, formulated as a paste, and used to bondsilicon wafers to alumina substrates. Ag₂ O was not disclosed. Thepastes were fired at 380° and 450° C.

OBJECTS OF THE INVENTION

It is an object of the invention to provide a die attach adhesivecontaining a glass characterized by a T_(g) not higher than about 300°C., good wetting properties, containing components which are compatiblewith melting in platinum crucibles to maintain high purity, andresistant to moisture and to recrystallization of components.

Another object is to provide two-piece ceramic articles bonded with alow-T_(g) PbO-B₂ O₃ -Ag₂ O glass.

A still further object is to adhere integrated circuit chips to aceramic base without degrading the chip.

An additional object is to reduce formation of NiO on substratemetallization.

Further objects are to provide particulate sizes of glass and Ag tominimize cracking and voids in the adhesive.

These and further objects will become apparent as the description of theinvention proceeds.

SUMMARY OF THE INVENTION

PbO-B₂ O₃ -Ag₂ O glass having compositions with a respective weightratio of 76-88:10-15:2-10 have glass transformation temperatures (T_(g))of less than 300° C. and can be used as adhesives for adhering togethertwo or more substrates such as for attaching integrated circuit chips toceramic substrates. The lower fusion temperatures avoid damage to thecircuits on the chips. Also, lower temperatures reduce NiO formation,which eliminates the need for hydrogen reduction prior to lid sealing anintegrated circuit into the package. In a preferred embodiment thecomminuted ("powderized") glasses can be mixed with silver particles toprovide thermally and electrically conductive adhesives for suchattachments.

When making the powdered glass and silver particle mixture, it has beenfound very effective to have the glass to Ag in a weight ratio of about1:3-5.7 and to have the Ag particles in a mixture of two components Aand B. Component A has Ag particles with an average size of about 9microns and with about 90 weight % less than 15 microns and about 10%less than 4 microns. Component B has Ag particles with an average sizeof about 3.5 microns and with about 90 weight % less than 11 microns andabout 10 weight % less than 0.7 micron. The weight ratio of componentsA:B preferably about 3-6:1.

Conductive adhesive pastes are made from the glass powders of thisinvention, conductive metal particulates (preferably Ag) and organicbinder components including solvents.

DETAILED DESCRIPTION OF THE INVENTION

The preferred procedure for the preparation of PbO-B₂ O₃ -Ag₂ O glasseson a small scale in 25 gm batches involves mixing powders of PbO (76-88wt. %), B₂ O₃ (10-21 wt. %), and Ag₂ O (2-10 wt. %) and melting inplatinum crucibles in an electrically heated furnace at 800° C. forabout 15 minutes. The glasses are formed by quenching the melts toambient temperature by pouring onto a steel platen. It was foundpreferable to add Ag₂ O in a different, non-oxide form such as AgNO₃ toform a homogeneous glass and to obtain the benefit of having a lowerT_(g). If Ag₂ O was used as such, the undesired reaction

    Ag.sub.2 O→2Ag+1/2O.sub.2

occurs during heating of the powder mixture, leading to Ag particulateinclusions in the glass and no substantial lowering of T_(g) since Ag₂ Ois not incorporated into the structure of the glass. ap The T_(g) of theglasses was determined by differential scanning calorimetry carried outat heating rate of 20° C./min in air. The results are shown in Table I.The glasses in all examples herein were prepared by the generalprocedure described above, using the amounts of ingredients as stated.

As can be seen in Table I, glasses in the PbO-B₂ O₃ -Ag₂ O systemexhibit a very low T_(g), particularly when Ag₂ O is substituted for PbO(while the B₂ O₃ content remains constant), and when Ag₂ O issubstituted for B₂ O₃ (while the PbO content remains constant). Theglasses of examples 12-14 have greater than 15% B₂ O₃ and are notpreferred glasses because the Tg temperature is greater than 300° C.

                  TABLE I                                                         ______________________________________                                        Variation of T.sub.g with Composition of Glasses                              Weight Percent                                                                Example                                                                       No.        PbO    B.sub.2 O.sub.3                                                                          Ag.sub.2 O                                                                          T.sub.g (°C.)                       ______________________________________                                        1          90     10         0     281                                        2          88     10         2     266                                        3          85     10         5     254                                        4          80     10         10    250                                        5          88     12         0     293                                        6          85     12         3     284                                        7          82     12         6     264                                        8          80     12         8     260                                        9          85     15         0     320                                        10         80     15         5     293                                        11         76     15         9     283                                        12         76     18         6     317                                        13         80     20         0     354                                        14         76     21         3     338                                        ______________________________________                                    

To illustrate further the effectiveness of Ag₂ O in PbO-B₂ O₃ glasses,PbO-B₂ O₃ mixtures having the same proportion of PbO and B₂ O₃ as in theglass of Example No. 6, but where the Ag₂ O has been replaced with otheradditives were prepared in the same way and the T_(g) determined.Results of the Comparison Examples 15-20, with various additves aregiven in Table II.

As can be seen from Tables I and II, the addition of Ag₂ O produced thelowest T_(g).

                  TABLE II                                                        ______________________________________                                        Influence of Additives on PbO--B.sub.2 O.sub.3 Glasses                        Example No. (a)    Additive T.sub.g (°C.)                              ______________________________________                                        (The Invention)                                                                6                 Ag.sub.2 O                                                                             284                                               (Comparison Example)                                                          15                 SiO.sub.2                                                                              323                                               16                 Al.sub.2 O.sub.3                                                                       314                                               17                 Bi.sub.2 O.sub.3                                                                       301                                               18                 BaO      298                                               19                 CdO      300                                               20                 ZnO      299                                               ______________________________________                                         (a) All examples used 85 wt. % PbO, 12% B.sub.2 O.sub.3, and 3% additive.

With a view of increasing resistance to crystallization, multicomponentglasses have been developed within the generic concept of the invention.It is expected that the addition of ZnO in an amount of up to 10 wt. %and the addition of Al₂ O₃ in an amount of up to 2 wt. % in PbO-B₂ O₃-Ag₂ O glass compositions can improve the resistance to crystallization.Examples are given in Table III. A preferred composition in Table III isshown in Example 23. That composition in particular has excellentstability towards devitrification, as indicated by the large temperaturedifference between T_(x) and T_(g). Tx is the temperature wherecrystallization first appears.

The resistance to moisture was tested by powderizing the glass andplacing it in a Parr bomb in water at 121° C. for 21 hours. The amountof extractable ions (in the water) from the glass of Example No. 23 wasfound to be much less than 25 ppm. See Table VI infra. This shows thecomposition had excellent resistance to moisture.

                  TABLE III                                                       ______________________________________                                        Composition of PbO--B.sub.2 O.sub.3 --Ag.sub.2 O -Based Glasses in Weight                                                 T.sub.x (°C.)              Example                                                                              PbO    B.sub.2 O.sub.3                                                                      Al.sub.2 O.sub.3                                                                    ZnO  Ag.sub.2 O                                                                          T.sub.g (°C.)                                                                (a)                               ______________________________________                                        21     83.42  11.61        1.64 3.33  279   398                               22     83.78  11.66        1.88 2.68  281   404                               23     83.71  11.45  0.29  1.88 2.67  280   410                               ______________________________________                                         (a) T.sub.x is determined from differential scanning calorimetry and is       defined as the temperature where cystallization first appears.           

ADHESIVE PASTES

Adhesive pastes can be made using the glasses of this invention bycomminuting the glasses such as the composition of Example No. 23 into apowder and optionally mixing with conductive particulates such as Agfollowed by mixing with an organic binder conventionally used for thispurpose. The glass comminution process can be done by conventionalprocesses such as by ball milling. However, particular attention shouldbe paid such that all the particles used for our invention pass througha 200-mesh screen and particles less than about 1-2 microns are notgenerated. In other words, if particles less than about 1-2 microns aregenerated, they should be discarded.

Glass particles finer than one or two microns will shrink excessivelyduring the firing of the adhesive paste. This results in voidingunderneath the die with consequently less heat dissipation from thedevice such as an integrated circuit. Excessive shrinkage also causescracks to form in the adhesive around the perimeter of the die. Thesecracks reduce both die shear and die pull strengths.

A homogeneous blend of two types of silver particles is preferably used.The silver used in larger amount is of an average particle size ofapproximately 9 microns. Its preferred particle size distribution isrelatively narrow, having approximately 90% less than 15 microns andabout 10% less than 4 microns. These coarse particles provide aconductive skeleton which shrinks minimally during the firing of theadhesive. It allows the glass to flow readily through its interstices,which in turn supplies its high fired strength.

The silver used in the smaller amounts is of an average particle size ofapproximately 3.5 microns. Its particle size distribution ranges fromabout 90% less than 11 microns to about 10% less than 0.7 micron. Theadvantage to this material is that it fits into the conductive skeletonmentioned before, while providing accelerated sintering at the reducedfiring temperature of this adhesive. This increases both the thermal andelectrical conductivity of the adhesive as well as its ultimatestrength.

The ratio of these two types of silver particles is chosen to minimizeunwanted shrinkage (e.g., by choosing a larger amount of the coarsesilver), while maximizing conductivity and strength (e.g., by choosingan increased amount of the finer silver). It has been found that theoptimum ratio of coarse to fine silver is about 4:1. Good adhesives canbe generated in ranges from about 6:1 to 3:1. So far as we candetermine, these silver blends are novel per se.

Various conventional organic binders are available, such as isobutylmethacrylate, nitrocellulose, cellulose acetate butyrate, etc. Apreferred organic binder is ethylcellulose. These materials areconventionally dissolved in a solvent for formulating the paste. Theamount of binder (exclusive of solvent) is typically about 0.2-1.0weight %, preferably about 0.35 weight %, based on the total weight ofpaste.

Various solvents may be used alone or in combination to generate thedesired evaporation rates. Ester-alcohols, terpene-alcohols, dibasicesters and glycol-ethers and their acetates are particularly suitablefor this application. These solvents have relatively high boiling pointsand, therefore, avoid unwanted room temperature volatility. Solvents arechosen for the combination of properties desired. Ester- andterpene-alcohols impart excellent strength and dispersibility. Dibasicesters and glycol ethers allow virtually void-free films upon drying.Typical solvent to suspend the solid particulates is in the range 9-24.8weight %.

Conductive adhesive pastes of our invention have the followingformulations:

(A) Glass powder of the invention, 11-23 weight %, preferably 15-19weight %;

(B) Conductive metal particulates (preferably Ag), 56-77 weight %,preferably 65-70 weight %; and

(C) Organic (including solvent), 10-25 weight %, preferably 13-17 weight%.

Our preferred paste uses the powdered glass of Example No. 23, one partby weight; Ag particulates, four parts; and organic, one part, where theorganic comprises ethylcellulose in an ester-alcohol solvent such asTexanol® (manufactured by Eastman Kodak). The Ag is a 4:1 blend of thetwo particle sizes as above described.

The paste may be applied to a ceramic base, and then the second ceramicitem to be attached such as an integrated circuit chip, may be appliedto the paste before firing. In the typical and most convenientapplication the paste is applied to a ceramic base such as alumina, andthen a second ceramic such as an integrated silicon circuit chip isapplied to the paste, and the assembly is heated to about 100° C. abovethe glass T_(g) point, whereupon a good bond results.

For example, bare-back silicon chips were attached to 96% aluminasubstrates using the paste of Example 23 at different temperatures. Theresults are set forth in Table IV.

                  TABLE IV                                                        ______________________________________                                        Composition of Examp1e No. 23                                                 Die Shear Strength Test (a)                                                   Peak Firing      Die Shear                                                    Temperature (°C.)                                                                       Strength (kg)                                                ______________________________________                                        380              18                                                           390              26.5                                                         405              38.5                                                         430              38.75                                                        ______________________________________                                         (a) Using 300 × 300 mil bareback silicon die on bare 96% alumina        substrates.                                                              

The die shear strength test which is described in MIL STD 883 Method2019.5 is essentially a measure of the shear force necessary to removethe die from the substrate to which it is adhered. A material would beconsidered accepted by the MIL specification if it had a die shear of 5kg. Since the values achieved here are over 3 times the specificationrequirement as reflected in die shear strength values shown in Table IV,they are considered to be bonded excellently at temperatures less than400° C.

Shear and pull strengths and ionic purity of a glass of this inventionas compared with a typical prior art glass was conducted as follows.

Glass was manufactured using the preferred embodiment of U.S. Pat. No.4,699,888 to Dumesnil et al. The glass composition was made from 5.44 gPbF₂, 0.90 g SiO₂, 2.18 g Cu₂ O, 1.46 g Bi₂ O₂, 5.44 ZnO, 1.18 g Al₂ O₃,8.16 g B₂ O₃ and 75.24 g PbO. Although Dumesnil et al discloses addingpowdered silver metal, there is no suggestion of the present compositionusing Ag₂ O. Dumesnil et al employ an 8-component glass system utilizingPbF₂, Cu₂ O and Bi₂ O₃ for lowering Tg. Bi₂ O₃, as mentioned before,attacks the platinum crucible used for melting of high purity glasses.PbF₂, as will be seen below, renders the glass to be less stable withrespect to attack by moisture. None of these components are present inour invention.

The T_(g) measured 269° C. for the Dumesnil et al composition ascompared to a glass from Example 23 of this application at 280° C.Although the T_(g) of the prior art material was lower than that of theglass of this invention, its fluidity when fired at 395° C. for sixminutes is less than that reported here as demonstrated by the followingstrength numbers achieved by substituting each glass into theformulation mentioned previously (i.e., glass 1 part, Ag 4 parts, andorganic 1 part, where the organic comprises ethylcellulose in anester-alcohol solvent):

                  TABLE V                                                         ______________________________________                                        Dumesnil      This Invention (Example 23)                                     ______________________________________                                        Die Shear*                                                                              11    kg    37         kg                                           Die Pull* 3     lb    50         lb                                           ______________________________________                                         *Using 0.300 × 0.300 inch bare backed silicon die on 96% alumina        substrates.                                                              

The die shear test is according to MIL STD 883 Method 2019 and the diepull test is according to MIL STD 883 Method 2027.

Additionally, ionic purity was measured on these two adhesive pastesmade with these glasses. Results were as follows:

                  TABLE VI                                                        ______________________________________                                                Dumesnil                                                                             This Invention (Example 23)                                    ______________________________________                                        F.sup.-   53 ppm   14 ppm                                                     Cl.sup.-  14 ppm   11 ppm                                                     Na.sup.+  15 ppm   15 ppm                                                     K.sup.+    6 ppm    3 ppm                                                     ______________________________________                                    

Ionic purity is generally required in die attach adhesives as it is inmost electronic devices. Residual moisture in the sealed package willcombine with available Na and K ions which can short the circuiting onthe chip. F and Cl ions when combined with moisture, form corrosiveswhich also degrade the device. Consequently, the lower the value for theconcentration of extractable ions shows that the composition of thisinvention is not only more resistant to water than the prior art, butalso better in terms of long-term device reliability.

The substrate-paste-chip assembly is preferably heated in a two stepprocess. First a drying step is used to drive off the organiccomponents, then the firing step melts the glass and bonds thesubstrates. Broadly speaking, both steps are well known in the art. Wediffer in that our glasses permit a lower firing temperature whilegiving a bond of superior properties. The initial drying step (to driveoff the organic) may be omitted under certain circumstances, forexample, if the die size is 0.05 inches square or less. Also, for largerdies, drying can be incorporated into the firing cycle by slowing theheating rate.

Laminates may be made by additional layers of paste and ceramic.

Considerable variation in the PbO-B₂ O₃ -Ag₂ O composition of the glassis possible while retaining low T_(g) and other desirable properties.Our work indicates that a PbO-B₂ O₃ -Ag₂ O weight range of76-88:10-15:2-10 is operable, and preferably 80-88:10-10-12:2-10. Ourexamples used 25 g. batches. Obviously, any size batch can be made up,limited only by the size of the platinum crucible and the heat source.Melt times may range from a few minutes to several hours, depending onbatch size, heat source, etc.

The invention glasses can be mixed with Ag (or any other conductiveparticulates) in a glass:conductive particulate weight range of1:2.3-7.3, preferably 1:3-5.7.

With respect to substrates adherable by the glasses of this invention,alumina and silicon ceramics are especially suitable. Others includeberyllia, steatite, silicon nitride, silicon carbide, gallium arsenide,etc.

The glasses of this invention are especially designed for adhesivefunction at about, or less than, 400° C., which temperature issufficiently low to prevent degradation of most integrated circuit chipsand Ni migration and oxidation. Obviously, if chips are being adheredwhere such degradation is not a problem, higher temperatures can beused, e.g., up to about 450° C.

In our examples we used Ag as the conductive matrix. However, ourinvention is not limited to Ag; substantially any of the conductiveparticulates conventionally used in the die adhesive art are useful,such as gold, platinum, or any of the air-fired metals. Nickel may beused with care at low temperatures.

It is understood that the foregoing detailed description is given merelyby way of illustration and that many variations may be made thereinwithout departing from the spirit of this invention.

What is claimed is:
 1. A homogeneous composition suitable for use informing a conductive adhesive comprisinga powdered glass compositionhaving a glass transformation temperature below about 300° C.,comprising PbO, B₂ O₃, and Ag₂ O in a respective weight ratio of76-88:10-15:2-10 and conductive particulates present in an electricallyconductive effective amount.
 2. A homogeneous composition according toclaim 1 comprising said powdered glass having a particle size whichpasses through a 200 U.S. mesh screen, but being not less than about 2microns and Ag particulates, in a glass:Ag weight ratio of about1:2.3-7.3.
 3. A homogeneous composition according to claim 2, whereinthe ratio is about 1:3-5.7.
 4. A homogeneous composition according toclaim 1 comprising said powdered glass and Ag particles, in a glass:Agweight ratio of 1:3-5.7; said Ag particles consisting essentially of amixture of components A and B;component A being Ag particles having anaverage size of about 9 microns, with about 90 weight % less than 15microns and about 10% less than 4 microns; component B being Agparticles having an average size of about 3.5 microns, with about 90weight % less than 11 microns and about 10 weight % less than 0.7micron; andthe weight ratio of components A:B is about 3-6:1.
 5. Aconductive adhesive composition according to claim 4, wherein theglass:Ag weight ratio is 1:4; and the ratio of components A:B is about4:1.
 6. A composition comprising the composition of claim 1 and furthercomprising an organic binder so as to form a paste composition.
 7. Acomposition comprising the composition of claim 2 and further comprisingan organic binder so as to form a paste composition.
 8. A compositioncomprising the composition of claim 3 and further comprising an organicbinder so as to form a paste composition.
 9. A composition comprisingthe composition of claim 4 and further comprising an organic binder soas to form a paste composition.
 10. A composition comprising thecomposition of claim 5 and further comprising an organic binder so as toform a paste composition.
 11. An electrically conductive pastecomposition comprising(A) 11-23 weight % powdered glass, said glasscomprising PbO, B₂ O₃, and Ag₂ O in a respective weight ratio of76-88:10-15:2-10; (B) 56-77 weight % conductive metal particulates; (C)10-25 weight % organic, including solvent.
 12. A paste compositionaccording to claim 11, wherein:(A) is 15-19 weight %; (B) is 65-70weight %; and (C) is 13-17 weight %.
 13. A paste composition accordingto claim 12, wherein (B) is Ag consisting of a homogeneous blend of (I)Ag particles having an average size of about 9 microns, with about 90weight % less than 15 microns and then about 10% less than 4 microns,and (II) Ag particles having an average size of about 3.5 microns, withabout 90 weight % less than 11 microns and about 10 weight % less than0.7 microns; the weight ratio of I:II being about 3-6:1.
 14. A pastecomposition according to claim 13, wherein I:II is about 4:1.
 15. Ahomogeneous composition according to claim 4, where said glassconsisting essentially of, in weight %, PbO 83.71%; B₂ O₃ 11.45%; Al₂ O₃0.29%; ZnO 1.88%; and Ag₂ O 2.67% and having a particle size whichpasses through a 200 U.S. mesh screen, but being not less than about 2microns.